There's many reasons.
First, cute design. Love hearts, blonde hair, blue eyes, and devil horn headband. It screams that she's a rebel, but a fucking cute one.
Second, personality. She's energetic and full of confidence (most of the time). Her heart is also in the right place the majority of the time.
Thirdly, she's flawed as hell. This makes her much more engaging as a character, so we relate to her more. It also makes us want to cheer her on so she overcomes the challenges in her way.
Star is a 10/10 character.